Everyone has covered the most important parts. I've learned that taking the time at the start of the project to cover there ideas, what they want the site/design to accomplish really helps in removing that "one more thing" annoyance.

Also being firm, even though it may be tough to do earns you way more respect and better clients as time goes on, however being firm should never be confused with being cocky/rude n etc towards a client. Word of such things can travel fast..but word of good work can and usually does travel faster.